Studies were performed in conscious dogs to determine if prostaglandins mediate the vasodilator action of hydralazine or modified its action in any way. Hydralazine was found to have a direct chronotropic effect on the heart which was enhanced by indomethacin and ibuprofen thus delaying the antihypertensive effect. This would suggest that hydralazine causes the release of a chronotropic agent whose release is modified by the prostaglandin system.